Mesophase pitch is an important and only relatively recently recognized member of the pitch family. Mesophase pitch has optical properties and can be used to make carbon fibers, carbon foam and other exotic and valuable materials.
When natural or synthetic pitches having an aromatic base are heated under quiescent conditions at a temperature in the range of 350° C. to 500° C., small insoluble liquid spheres begin to appear in the pitch and gradually increase in size as heating is continued. When examined by electron diffraction and polarized light techniques, these spheres are shown to consist of layers of oriented molecules aligned in the same direction. As these spheres continue to grow in size as heating is continued, they come in contact with one another and gradually coalesce with each other to produce large masses of aligned layers. As coalescence continues, domains of aligned molecules much larger than those of the original spheres are formed. These domains come together to form a bulk mesophase wherein the transition from one oriented domain to another sometimes occurs smoothly and continuously through gradually curving lamellae and sometimes through more sharply curving lamellae. The differences in the orientation between the domains create a complex array of polarized light extinction contours in the bulk mesophase corresponding to various types of linear discontinuity in the molecular alignment. The ultimate size of the oriented domains produced is dependent upon the viscosity and the rate of increase of the viscosity of the mesophase formed, which in turn is dependent upon the particular pitch and the heating rate. In certain pitches, domains having sizes in excess of two hundred microns up to in excess of one thousand microns are produced. In other pitches the viscosity of the mesophase is such that only limited coalescence and structural rearrangement of layers occur so that the ultimate domain size does not exceed one hundred microns.
The highly oriented, optically anisotropic insoluble material produced by treating pitches in this manner has been given the term “mesophase”, and pitches containing such material are known as “mesophase pitches”. Such pitches when heated above their softening points are mixtures of two essentially immiscible liquids, one optically anisotropic oriented mesophase portion and the other the isotropic non-mesophase portion. The term “mesophase” is derived from the Greek “mesos” or “intermediate” and indicates the pseudo-crystalline nature of this highly oriented, optically anisotropic material. Mesophase is in essence a “liquid crystal” since it has an orderly and repeating arrangement of its atoms as evidenced by its X-ray diffraction pattern and yet is capable of flow when stress is applied. This seemingly contradictory behavior results from the rather weak bonding of carbon atoms in adjacent parallel planes.
In a sense, mesophase pitch is just a stopping point along the way of thermal condensation of hydrocarbons into coke. As time and temperature increase, aromatic liquid hydrocarbons thermally polymerize with some thermal de-alkylation. If the atmospheric or vacuum residual of an aromatic crude oil is thermally treated, the first stop is vis-broken crude with a lower viscosity and lower molecular weight than the feed. The next stop along the thermal treatment route is dominated by thermal polymerization yielding petroleum pitch. The end of the line is coke. Mesophase pitch is the penultimate stop. Although the thermal processes can be briefly explained, myriad processes for making mesophase have been proposed or at least patented.
To better explain our new process, a review of generic information about pitch follows. This review covers a discussion of end products, patents about producing mesophase including our prior patents on the topic, and a discussion of multiple types of mesophase.
The term pitch has been used for many heavy products, ranging from a residual fraction of crude oil to the product of thermal polymerization. As used herein, pitch is intended to refer to the highly aromatic material with a softening point greater than 100° C. produced by thermal polymerization.
Petroleum pitches have been made for decades by refiners. Perhaps the most widely known material is A-240 pitch and/or M-50 produced by Ashland Petroleum Company and subsequently Marathon Oil Company respectively. Such pitches with suitable softening points can be used satisfactorily as an impregnation material for electrodes, anodes, and carbon-carbon composites, e.g., carbon-carbon fiber composites, such as aircraft brakes and rocket engine nozzles. These pitches can also be used in the nuclear industry for the preparation of fuel sticks and control rods for a graphite moderated reactor. Furthermore, such pitches can be used as a starting material for the production of mesophase pitch which can be used in the production of carbon fiber precursors and carbonized fibers, i.e. carbon fibers and graphite fibers. Carbon foams and other pitch-based products can be made as well from mesophase pitch.
High strength per weight ratio of carbon and graphite fibers, alone or in composites, makes such fibers useful in sporting equipment, automobile parts, light-weight aircraft, and several aerospace applications. High thermal conductivity and strength make carbon foam useful for thermal management applications and more. The end products, carbon fiber, carbon foam and the like, are high value specialty products which rely heavily on the properties of the starting material, the mesophase pitch.
A more or less chronological review of pitch preparation and pitch use patents follows. In U.S. Pat. Nos. 3,974,264 and 4,026,788, McHenry discloses producing carbon fibers from pitch. A non-thixotropic spinnable mesophase pitch having a mesophase content in the range of about 40 wt % to about 90 wt % is produced with shorter processing time by passing an inert gas through the pitch at a temperature in the range of 350° C. to 450° C.
In U.S. Pat. Nos. 3,976,729 and 4,017,327, Lewis, et al. disclose preparation of a non-thixotropic mesophase pitch while agitating the pitch during formation of the mesophase in order to produce a homogeneous emulsion of the immiscible mesophase and non-mesophase portions of the pitch. Improved rheological and spinning characteristics result from heating the pitch in an inert atmosphere at a temperature in the range of 380° C. to 440° C. for a time sufficient to produce a mesophase content in the range of 50 wt % to 65 wt % while agitating the pitch during the formation of the mesophase. A smaller differential between the average molecular weights of the mesophase and non-mesophase portions of the pitch also occurs.
In U.S. Pat. No. 3,995,014, Lewis discloses subjecting pitch to a reduced pressure during formation of the mesophase in order to substantially reduce the time otherwise required for its preparation.
In U.S. Pat. No. 4,005,183, Singer discloses a process for forming high-modulus, high-strength carbon fibers having a highly oriented structure containing crystallites. A mesophase-containing fiber is heated in an oxygen-containing atmosphere at 250° C. to 400° C. for a time sufficient to render it infusible, and then heated in an inert atmosphere to at least 1,000° C.
In U.S. Pat. No. 4,080,283, Noguchi, et al. disclose continuous production of pitch from a heavy hydrocarbon oil by mixing with an inactive gas such as nitrogen or steam, and heating the mixture at a temperature between 350° C. to 500° C. serially in a plurality of reactors with a portion of the liquid output from at least one of the reactors being re-circulated. The liquid output of the final reactor can be charged to an after-treatment duct-shaped chamber with an inactive atmosphere to cool said liquid output. Such operation provides uniformity of reaction conditions in the reactor system.
In U.S. Pat. No. 4,184,942, Angler, et al. disclose producing an optically anisotropic, deformable pitch from a carbonaceous isotropic pitch by initially heating at between 350° C. to 450° C. and then extracting with an organic solvent system. The solvent-insoluble fraction can be converted into an optically anisotropic pitch.
In U.S. Pat. No. 4,208,267, Diefendorf, et al. disclose producing an optically anisotropic, deformable pitch from the solvent-insoluble fraction of a carbonaceous isotropic pitch that has been extracted with an organic solvent, e.g. such as benzene or toluene. The solvent-insoluble fraction is heated for ten minutes or less to temperatures between 230° C. to 400° C. to yield an optically anisotropic phase of greater than 75 wt %. The phase contains less than about 25 wt % of substances unextractable with quinoline at 75° C.
In U.S. Pat. No. 4,209,500, Chwastiak discloses producing both a single-phase, essentially 100% anisotropic mesophase pitch having number average molecular weight below 1000, a net pyridine insoluble content no greater than 60% by weight, a softening temperature no greater than 350° C., and a viscosity no greater than 200 poises at 380° C. and carbonaceous fibers therefrom. An inert gas is passed at a sufficient rate through an isotropic carbonaceous pitch while heating said pitch at between 380° C. to about 430° C. to agitate the pitch sufficiently to produce a homogeneous emulsion of the mesophase and to ensure removal of volatile low-molecular weight components. “Inert gas” is meant to be a gas which does not cause a significant change in the chemical nature of the pitch materials being contacted at the process conditions of temperature and pressure.
In U.S. Pat. No. 4,402,928, Lewis, et al. disclose producing a carbon fiber from precursor material such as ethylene tars, ethylene tar distillates, gas oils derived from petroleum refining, gas oils derived from petroleum coking, aromatic hydrocarbons, and coal tar distillates having at least 50% by weight which boils under about 300° C. and at least about 70% by weight which boils under 360° C. One of these precursor materials is heated in batches under pressure to obtain a pitch which is solvent extracted to obtain a 70% by weight or greater mesophase portion. The insoluble mesophase portion can be converted into a carbon fiber.
In U.S. Pat. No. 4,460,557, Takashima, et al. disclose producing carbon fibers by heating a pitch to between 340°-450° C. under a stream of inert gas, such as nitrogen at up to atmospheric pressure, melt spinning the resulting material to form pitch fibers, then infusibilizing and carbonizing or graphitizing them.
In U.S. Pat. No. 4,504,455 and European Patent Application 813058930, Publication No. 0054437, Otani, et al. disclose a carbonaceous pitch comprising quinoline-soluble dormant anisotropic hydrocarbon components that are partially hydrogenated mesophase portions of a mesophase pitch. The carbonaceous pitch is optically isotropic in nature with a dormant mesophase which is orientable when subjected to shear forces. The dormant mesophase pitch is prepared by hydrogenating the mesophase of a mesophase pitch until substantially all the mesophase is quinoline-soluble. Production of a carbon fiber from these pitches is also disclosed. In the European application, the dormant mesophase pitch is prepared by solvent extracting mesophase pitch into quinoline-insolubles and quinoline solubles and then hydro-treating the quinoline insoluble portion. The higher the measured quinoline-insoluble fraction, the higher the amount of mesophase components present.
In U.S. Pat. No. 4,528,087, Shibatani, et al. disclose producing with extraction, a mesophase pitch containing 40% or more of quinoline-soluble. A pitch with an aromatic hydrogen content of 50% to 90% is heated to 430° C.-550° C. while passing an inert gas thereover until at least 40% mesophase is formed.
In U.S. Pat. No. 4,529,498, Watanabe discloses producing a 100% mesophase pitch of quinoline-insoluble and quinoline-soluble components by (1) heating a petroleum derived pitch to a temperature of 360°-450° C. while stirring under a low molecular weight hydrocarbon gas atmosphere at atmospheric or super-atmospheric pressure until the mesophase content is 10% to 50% to form a heat treated pitch, (2) holding without stirring the heat treated pitch at a temperature in excess of 280° C., but below 350° C., to permit separation into a layer of non-mesophase and a layer of mesophase, and (3) separating the non-mesophase layer from the mesophase layer. High-strength, high-modulus carbon fibers can be produced from the resulting mesophase layer.
In U.S. Pat. No. 4,529,499, Watanabe adds to the method of U.S. Pat. No. 4,529,498 by subjecting separated non-mesophase material to steps (1), (2), and (3) at least 3 times to prepare a 100% mesophase composed only of quinoline-insoluble and quinoline-soluble components.
In U.S. Pat. No. 4,575,411, Uemura, et al. disclose producing a melt-spinnable carbon fiber precursor pitch with a softening point between 200° C. to 280° C. by heating a film of 5 mm or less of a carbonaceous pitch at a temperature of 250° C. to 390° C. and at a pressure of 100 mm Hg or less until the precursor pitch contains 40% or more mesophase material. The mesophase pitch has 0 wt % to 40 wt % of an anisotropic quinoline-insoluble phase and 85 wt % to 100 wt % of an anisotropic quinoline-soluble phase.
In U.S. Pat. Nos. 4,497,789 and 4,671,864, Sawran et al. disclose producing substantially non-mesophasic pitch with a wiped-film evaporator.
In U.S. Pat. No. 4,976,845, Oerlemans, et al. disclose producing mesophase pitch using a wiped film evaporator.
In U.S. Pat. No. 5,238,672 and a Division thereof, U.S. Pat. No. 5,614,164, Sumner et al. disclose agitating heavy isotropic pitch at a temperature in the range of about 327° C. to about 454° C. for a time sufficient to produce a mesophase pitch having a minimum mesophase content of about 60 vol. %. An example showed distilling a commercially available isotropic pitch, A-240, in a wiped film evaporator to make a heavy isotropic pitch. This heavy pitch was converted to mesophase by gentle heating in a stripping vessel to 404 C and agitation with bubbling nitrogen gas for 4½ hours.
Chwastiak's method in U.S. Pat. No. 4,209,500 involving stripping requires a relatively long time to obtain mesophase spinnable pitch from a base pitch. Not only is stripping time consuming, but also high-molecular weight materials can be carried over with low-molecular weight materials during stripping due to foaming and the like. The volatile carry over from stripping may lose potentially useful components hard to recover due to the presence of highly diluting stripping gases and highly cracked materials that increase with residence time at high temperatures.
The method of Diefendorf, et al. in U.S. Pat. No. 4,208,267 involves a solvent extraction to remove low-molecular weight component which is rather difficult to practice.
Carbonaceous materials (sometimes called fiber precursors) for the manufacture of carbon or high-strength graphite fibers, conventionally employ polyacrylonitrile or mesophase pitch. However, preparation of mesophase pitch requires a time-consuming and expensive batch process of heating at an elevated temperature for a number of hours, as shown by Lewis, et al. in U.S. Pat. No. 3,967,729, by Singer in U.S. Pat. No. 4,005,183, and by Schulz in U.S. Pat. No. 4,014,725. Improper heating can increase viscosity of mesophase pitch so much that it is rendered unsuitable for spinning. Also, polyacrylonitrile is often a more expensive feedstock than is mesophase pitch.
In U.S. Pat. No. 6,833,012, Rogers reviews methods of making mesophase pitch.
Pitch formation is a thermal process involving thermally induced polymerization. The product has a higher molecular weight than the feed. In contrast, there are other thermal refinery processes which use heat to crack or dehydrate the feed. These processes produce products with lower molecular weights than the feed. Thermal cracking processes such as visbreaking, e.g., a thermal cracking process widely licensed by Universal Oil Products, used high temperature to thermally crack high molecular weight components of crude oil to create its own cutter stock, reducing the viscosity of the heavy fuel oil product. Steam cracking of naphtha or other light, usually paraffinic, feeds to olefins is an important method to produce ethylene and other light olefins. Steam and naphtha are mixed together and fed through a heater at ultra high temperatures as high as 850° C. and at velocities exceeding the speed of sound, then quenched. Styrene production, although catalytic, uses large amounts of superheated steam to heat an ethylbenzene feed to a temperature where it can catalytically and endothermically be converted to styrene.
The state of the art on making mesophase pitch could be summarized as follows. There are many processes most involving relatively long batch processes which allow mesophase to form. Some are continuous and use intense mechanical agitation after using a wiped film evaporator to remove a substantial amount of distillate material or agitation by injection of an inert gas. All are difficult to control and, because the temperatures are high, the mesophase pitch precursor and the pitch product can form coke. Mesophase formation is generally enhanced by low pressure to strip off lighter byproducts or relatively light materials which may be present during thermal polymerization. These processes require residence times of hours up to days to produce the desired mesophase product.
Our recent patent activity will be reviewed next.
Our first U.S. Pat. No. 7,220,348 teaches a way to use superheated steam to effectively steam strip pitch in order to produce a higher softening point pitch with an ultra low residence time of less than 1 second. No mesophase production was reported in the examples. Our second U.S. Pat. No. 7,341,656 teaches use of steam and an oxidant to produce mesophase pitch.
U.S. Pat. No. 7,220,348, Malone et al, assigned to Marathon Ashland Petroleum LLC, taught a way to make a high softening point pitch. The examples showed contacting an A-240 pitch with superheated steam. The softening point of the pitch was increased, indicating that something akin to steam stripping or removal of lighter components by distillation had occurred. Although the term mesophase was used, and even included in the examples and Tables reporting experimental results, all examples showed that at the conditions used “nil” mesophase was found in the product.
U.S. Pat. No. 7,341,656, Malone et al, assigned to Marathon Ashland Petroleum Co LLC, was for a CONTINUOUS OXIDATION AND DISTILLATION PROCESS OF HEAVIER HYDROCARBON MATERIALS. The patent is primarily directed to an improvement on a used lube oil re-refining process, but does mention other feeds such as slurry oil, asphalt or petroleum pitch. The process heats the heavy feed by contact with superheated steam and an oxidizing gas. Combustion heats the feed, promoting fractionation. The heavy feed, steam and oxidant are mixed in a nozzle and discharged into a vessel. The process conditions include “a superficial velocity of no greater than about 5.5 feet per second, preferably no greater than about 3 feet per second.” Velocities are limited to limit entrainment of liquid. In Example 1, the only example in the patent, pressure is not reported, however the patent reports that the “steam-light overhead mixture was cooled first to 225° F., where most of the overhead product condensed. The steam was condensed and collected in a water condensate accumulator.” From the temperature reported, i.e. 225° F., the pressure was slightly above atmospheric. The teachings of this patent could be summarized as steam stripping plus oxidation may be used to heat and volatize light components from used motor oil. Other streams may also be heated.
While there is a voluminous amount of art on making mesophase pitch, none have been completely satisfactory. The reaction is simple—thermal polymerization and usually some thermal de-alkylation, but difficulties have been encountered in the past in trying to make a reliable process. It is easy to make mesophase pitch from any aromatic containing starting material—every delayed coker forms mesophase and promptly turns it into low value coke. Slow processing by working with a temperature just at the threshold of that needed to induce thermal polymerization and careful control of temperature has worked to some extent in the past. The slow processing gives some control over the process. Other processes used high temperatures and intensive mechanical working using wiped film evaporators to improve heat transfer, limit the residence time at high temperature, and prevent stagnant regions to minimize coke formation.
We wanted to develop a simple but robust process which did not require complicated expensive mechanical equipment and which was not prone to fouling by coke formation. We did not want to resort to an in-situ combustion approach to generate the high temperatures needed for mesophase formation because this complicates the design of the plant and may impact the quality of the products and certainly of the by-products which are burned.
We were doing some experiments in the laboratory which were somewhat related to our prior patents that used steam, or steam and an oxidant to distill pitch. We used a long tube reactor and relatively severe thermal conditions at slightly above atmospheric pressure to keep more of the feed components in liquid phase. We discovered that it was possible in a long tube reactor to create significant amounts of mesophase pitch even with short residence times on the order of 0.1 second. Others had converted petroleum pitch with a softening point of about 240° F. to mesophase, but they required a residence time of hours to days. They were able to shorten the residence time to an hour or two using intense mechanical agitation after processing in a wiped film evaporator. In summary, we were able to make mesophase pitch in orders of magnitude less time, using a simple long tube or pipe reactor. Processing conditions were fairly intense. It is hard to determine exactly what flow regime occurred in the pipe, it may have been fully developed turbulent mist-annular flow. It is possible that some, or perhaps even much or all, of the flow in the tube was spray or mist annular. The important factor is to have intense mixing, with the intensity was provided by fluid dynamics rather than mechanically.
This discovery—that significant amounts of mesophase could be made in less than one second if the temperature was high enough and the conditions turbulent—was the starting point on our new route to mesophase pitch.
Our starting material can be identical to the starting material of most pitch processes, e.g., a conventional petroleum pitch. We used A240, a widely used product that is no longer commercially made. Our approach, however, is different from other processes. Rather than take a long time to make mesophase in a batch reactor or a somewhat shorter time in a wiped film evaporator, we make mesophase in seconds without any mechanical agitation. Rather than operate under vacuum, we prefer to operate near atmospheric pressure. Other processes used vacuum to remove light materials, but in our process steam is preferably added. Much of the heat required is provided by injecting superheated steam, but to keep the amount of steam injection low, we prefer to add additional heat by conducting the reaction in a heated long tube. This long tube is heated preferably with electric resistance or inductance heating, or by immersing the long tube in a salt bath or the like or by putting the tube in a fired heater. Uniform precise temperature control is helpful, but the flows through the tube are at such high velocity and the conditions within the tube so turbulent, that an ascending or descending or some other temperature profile can be used, if desired.
By using unusually high velocities in the long tube thermal reactor, we were able to create conditions which fostered rapid mesophase formation. Additionally, the flow regime was so vigorous that it was possible to operate the long tube reactor for a significant time without coke formation despite the high temperatures used.
In addition to being a new route to mesophase, our new mesophase pitch may be a new composition of matter. As noted by Dr. James Klett, two materials can be called mesophase pitch but have significantly different molecular structures and different properties. Mesophase pitch made from naphthalene, Mitsubishi AR, has a softening point of 273° C. and a lower carbon yield of 78% as compared to a mesophase pitch made by a proprietary Conoco process from petroleum pitch which has a melting point of 355° C. and a carbon yield of 87%. (www-physics.lb1.gov/˜gilg/ATLASUpgradeRandD/HighKFoam/Graphite_Foams.pdf)
Mesophase pitch produced by the invention described herein has been analyzed to have a softening point (ASTM D3104) of 323° C., a mesophase content (ASTM D4616) of 82% vol, a quinoline-insoluble (QI) content (ASTM D2318) of 34.7% and a coking value (ASTM D2416) of 90%. Mesophase pitches with different properties can be made by operating the instant invention at different conditions. It is remarkable that a mesophase pitch with a mesophase content of 82% and a coking value of 90% would have a QI of less than 35%. Typically the QI for such a pitch would be 55% or higher. Generally a low QI for a given mesophase content is considered to be highly desirable.